I've built a skeleton cello (680mm scale length, 79mm fingerboard radius) and due to the high price of cello strings (2-400$ a set?), I found some flatwound bass strings whose string gauges seem to match well (0.100 to 0.045).

Tuning these up to pitch continues to break them. The tension on the strings is immense, more than I feel safe putting on a string (which is why they are breaking, I assume).

Looking at cello strings, they are pretty much the same. A core wound with a (flat) winding. Sure they may be built to produce a better cello tone, but that's not what I'm concerned with right now.

So what's so special about cello strings that allows them to be tuned to pitch without having to be put under so much tension? Is it a thicker/thinner core? All of my physics knowledge points to this making no sense - a string of a given mass must be under a given tension to vibrate at the selected pitch.